Monolithic integration may be not feasible because it may be either technically not possible or not cost effective.
An example where the monolithic integration may not be a cost-effective option is the monolithic integration of a Complementary-Metal Oxide Semiconductor (CMOS) interface on a MEMS (Micro Electro-Mechanical) sensor, which often requires running through a costly CMOS process which won't cover the entire substrate area.
The monolithic integration may be technically not viable because either the substrate onto which the devices are to be integrated cannot withstand the process conditions (e.g. high temperature steps), the required material cannot be deposited with sufficient quality onto the foreign substrate (e.g. due to structural incompatibilities) or the process flow may be incompatible with devices previously fabricated on the receiver substrate (e.g. high temperature steps after metallisation of previous devices or contamination issues).
Display technologies are an example where the structural incompatibilities in conjunction with the low thermal budget of the glass substrate inhibit the formation of single-crystalline semiconductors on amorphous glass substrate and where it is advantageous to integrate high performance semiconducting devices with differing functionality. Examples of such devices include npn transistors and pnp transistors (e.g. to form CMOS circuits), pressure sensors (e.g. for haptic interfaces), light sensors (e.g. for adapting the display to the ambient lighting conditions) and last but not least red, green and blue Light Emitting Devices (LEDs) (e.g. for emissive displays) on a transparent substrate such as a glass substrate or plastic substrates which may be flexible.
These devices may contain elongate low dimensional structures, which are formed onto a suitable substrate but can be subsequently transferred onto a different substrate. Examples of devices which may contain elongate low dimensional structures are npn transistors, pnp transistors, sensors, capacitors, red, green and blue LEDs. The bulk of the receiver substrate may consist of glass, polymers, metals, or semiconductors.
Where physically different structures consisting of or containing low dimensional structures are formed on a formation substrate and are transferred to a target substrate, it is desirable to be able to exercise a degree of control over the arrangement of these devices on the target substrate after transfer/re-orientation, both with respect to predefined features on the target substrate and with respect to each other.
The term “low dimensional structure” as used herein refers to a structure that has at least one dimension that is much less than at least a second dimension.
The term “elongate structure” as used herein refers to a structure having at least two dimensions that are much less than a third dimension. The definition of an “elongate structure” lies within the definition of a “low dimensional structure”, and a nanowire is an example of a structure that is both a low dimensional structure and an elongate structure.
Low dimensional structures that are not elongate structures are known. For example, a ‘platelet’, which has two dimensions of comparable magnitude to one another and a third (thickness) dimension that is much less than the first two dimensions constitutes a “low dimensional structure” but is not an “elongate structure”.
For the avoidance of any doubt, the term “physically different” means in this context that those sections of the elongate low dimensional structure which determine the device performance differ in at least one of the following points:
1. material composition—e.g. doping concentrations, doping type (p and n doped regions), semiconducting material with different band gaps;
2. material composition profile—e.g. doping profiles along the structure and/or presence of heterojunctions;
3. cross-sectional geometry—e.g. different side facets at different sections along the low dimensional elongate structures;
4. density of elongate low-dimensional structures—e.g. the elongate low-dimensional structures may branch into several elongate low-dimensional structures, or some of the elongate low-dimensional structures may be shorter than others;
5. orientation of elongate low-dimensional structures—e.g. the elongate low-dimensional structures may change their orientation at well defined positions along their lengths (kinks); and
6. cross-sectional dimensions of elongate low-dimensional structures—e.g. different cross sectional areas of the elongate low dimensional structures.
Additionally, the physically different sections may also differ in their length.
Methods are known for transferring structural features from a first substrate to a second substrate. However, at present no suitable techniques are available for applying a high density of structural features with an elongate/low dimensional geometry to a receiver substrate such that all of the following desiderata can be met:
1. The elongate features consist of at least two different segments/regions, which are distinguishable from each other by different variations in their material composition along their longest dimension.
2. The elongate features are transferred to a target substrate with one and only one transfer step.
3. The spatial arrangement and spacing of the elongate/low dimensional structures within devices defined on the target substrate can be substantially controlled.
4. The elongate low dimensional structures are oriented on the target substrate such that any arbitrary symmetric or asymmetric distribution of the material composition progresses along their longest dimension in the same manner for all structures.
5. At least one edge of the elongate low dimensional structures is aligned with one or more common planes.
6. Physically different devices containing the same number of elongate low dimensional structures with cross sections which scale in a substantially similar manner can be obtained in close proximity independent of yield and reproducibility issues related to the fabrication of the elongate low dimensional structures.
Control over one or more (and preferably all) the factors set out above is necessary to permit the use of such elongate or low dimensional structures to improve existing and develop new nanotechnologies.
U.S. Pat. No. 7,067,328 discloses a method for transferring nanowires from a donor substrate (for example the substrate on which they are formed) to a receiver substrate. This is achieved by disposing an adhesion layer on the receiver substrate, and mating it with the donor substrate. A degree of alignment and ordering of the nanowires on the receiver substrate is achieved by moving the donor substrate and receiver substrate relative to one another while they are in contact.
U.S. Pat. No. 6,872,645 teaches a method of positioning and orienting elongate nanostuctures on a surface by harvesting them from a first substrate into a liquid solution and then flowing the solution along fluidic channels formed between a second substrate and an elastomer stamp. The nanostructures adhere to the second substrate from the solution with a preferred orientation corresponding to the direction of fluid flow.
U.S. Pat. No. 7,091,120 discloses a process in which a liquid material is disposed on a population of nanowires that are attached to a first substrate with their longitudinal axes perpendicular to the plane of the first substrate. The material is then processed in order to cause it to solidify into a matrix that is designed to adhere to the nanowires and act as a support for the nanowires during the process of separating the nanowires from a first substrate and transferring them to a second substrate. Optionally, once the composite of nanowires embedded in the matrix material has been successfully transferred to the second substrate the matrix material can be removed to leave only the nanowires.
U.S. Pat. No. 7,091,120 also discloses an extension to this process whereby the composite of nanowires embedded in the matrix material is lithographically patterned into blocks. The blocks are then applied to a second substrate such that the embedded nanowires are aligned with their longitudinal axes parallel to the plane of the second substrate.
In one embodiment of the method of U.S. Pat. No. 7,091,120 the composite material is formed by unidirectionally disposing the matrix material on an ordered or random arrangement of nanowires. The directional flow of the matrix material induces the nanowires to orientate within the composite material parallel to the plane of the first substrate.
The method of U.S. Pat. No. 7,091,120 has a number of disadvantages, as follows:                Deposition of the matrix as a liquid may disturb the alignment/orientation of the elongate nanostructures on the donor substrate. Hence, it is challenging to control the arrangement and/or orientation of the elongate structures contained in each block relative to the external dimensions of the block.        The absolute dimensions and aspect ratio of the composite blocks are limited by the resolution, alignment accuracy and anisotropy of the lithographic and etch processes used to pattern the blocks (generally, only blocks with a low aspect ratio can be obtained). Consequently, it is difficult to control the number of elongate structures contained in each block or, again, the arrangement of elongate structures contained in each block relative to the external dimensions of the block.        The method does not easily enable nanostructures to be reoriented from a perpendicular orientation relative to the first substrate to a parallel orientation relative to the second substrate.        
US patent application No. 2004/0079278 discloses a method of forming a composite material comprising an array of isolated nanowires and a matrix that fills in the gaps between the materials. This method is designed to fabricate monolithic photonic band gap composite structures that cannot easily be transferred between different substrates.
U.S. Pat. No. 7,068,898 discloses a composite structure comprising nanostructures dispersed in a polymer matrix with random and ‘less random’ orientations. The application is directed to light concentrators and waveguides that take advantage of the anisotropic emission pattern to ensure light is redirected in the guide or concentrator as desired.
Small, Vol. 1, No. 1, p. 142 (2005) describes how three LEDs were fabricated using one uniformly p-doped nanowire which is crossed by three identical n-doped nanowires forming three pn junctions. Each pn junction emits light at the same wavelength. This publication suggests the assembly of three pn junctions each emitting light at different wavelength by replacing the three identical n-doped nanowires with nanowires consisting of three different suitable materials (GaN, CdS, and CdSe). The same approach of assembling crossing nanowires is used to demonstrate the integration of one LED with one FET, where the difference in functionality is solely achieved by using different operation conditions (voltages). Science, Vol. 294 p. 1313 (2001) uses the same assembly approach as Small, Vol. 1, No. 1, p. 142 (2005) to realise logic gates.
In both cases, the technology described requires two assembly steps to fabricate a cross-bar arrangement. Even if one could envisage a similar approach by replacing the p-doped bottom nanowire with patterned p-doped Si, the transfer and assembly technology described (a fluid assembly method) would not allow to obtain devices consisting of different material compositions in well defined spots using only one transfer step. Furthermore, it would be impossible to assemble nanowires with asymmetric doping profiles with identical orientation. Therefore, the device performance of each group of devices can't be optimised independent of each other unless several transfers are performed. Also, the method is not suited to assemble nanowires with asymmetric doping profiles which are often desired if the operating conditions (voltages) are asymmetric as in the case of transistors.
Proceedings of the IEEE, Vol. 93, No. 7, p. 1357 (2005) describes a logic gate realised by using a single, uniformly doped nanowire. Similar to the method described in Small, Vol. 1, No. 1, p. 142 (2005), the logic operation requiring different devices (e.g. resistor and transistor) is achieved by applying different operating conditions (voltages) to different segments of the nanowire. This approach suffers from the same drawbacks described in the previous publications as far as the device optimisation by different and asymmetric doping profiles is concerned.
Co-pending UK patent application No. 0620134.7 (UK patent application publication No. GB2442768A) describes a method of making encapsulated low dimensional structures such that they are suitable to be transferred to a different substrate. During the transfer the number of elongate structures, their alignment, spacing, and their orientation are maintained. Furthermore, these structures can be subsequently processed into devices using conventional lithographic methods in combination with subtractive (e.g. dry etching) and additive techniques (e.g. metal deposition). The number of elongate structures within each device is well controlled. It is possible to divide each group after the transfer into smaller segments to yield multiple identical devices out of each group (e.g. several npn transistors).
US2005/0180194 discloses a “nano tube cell” containing alternating regions of p-type doping and n-type doping. This cell may be incorporated in a structure such that two PNPN diode switches are defined in the tube cell. US2005/0180194 does not describe in detail how the structure is fabricated. The structure shows a symmetric IV curve, which implies that the two diode switches are identical to one another.
US2007/0102747 relates to a carbon nano tube FET (CNTFET) structure, and proposes a structure incorporating an n-type FET and a p-type FET. However, the two FETs are physically identical, and the n-type FET and p-type FET are obtained by applying different gate voltages to the two structures.
US2003/0089899 describes formation of regions of different doping type in a nanoscale wire to produce a single device.